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| United States Patent |
6,250,812
|
|
Ueda
, et al.
|
June 26, 2001
|
Rolling bearing
Abstract
The present invention provides a rolling bearing having an excellent
corrosion resistance and toughness which can fairly operate at a high
rotary speed. At least the inner race is formed by a titanium alloy, and
the rolling elements are formed by ceramics. Alternatively, at least one
of the inner race and the outer race is formed by a .beta. type titanium
alloy. The percent cold working of the .beta. type titanium alloy is
predetermined to not less than 20% or a range of from 5 to 20%. The cold
working is followed by shot peening. Further, the surface hardness Hv is
predetermined to not less than 600. The volumetric ratio of residual
.beta. phase in the .beta. type titanium alloy is predetermined to a range
of from 30 to 80%.
| Inventors:
|
Ueda; Kouji (Kanagawa, JP);
Ohori; Manabu (Kanagawa, JP)
|
| Assignee:
|
NSK Ltd. (Tokyo, JP)
|
| Appl. No.:
|
108391 |
| Filed:
|
July 1, 1998 |
Foreign Application Priority Data
| Jul 01, 1997[JP] | 9-188898 |
| Dec 02, 1997[JP] | 9-345718 |
| Apr 03, 1998[JP] | 10-107102 |
| Current U.S. Class: |
384/492; 384/609; 384/912 |
| Intern'l Class: |
F16C 019/06 |
| Field of Search: |
384/492,565,569,912,609
|
References Cited
U.S. Patent Documents
| 5086560 | Feb., 1992 | Glazier | 384/548.
|
| 5518820 | May., 1996 | Averbach et al. | 384/492.
|
Primary Examiner: Footland; Lenard A.
Attorney, Agent or Firm: Crowell & Moring, L.L.P.
Claims
What is claimed is:
1. A rolling bearing comprising races composed of an outer race and an
inner race and rolling elements which are provided between the outer race
and the inner race such that the rolling elements rotate freely, wherein
at least said inner race is made of a titanium alloy and said rolling
elements are corrosion resistant, wherein said titanium alloy is selected
from the group consisting of a .beta. type titanium alloy and an
(.alpha.+.beta.) type titanium alloy and said rolling elements are made of
a material selected from the group consisting of ceramics and martensite
stainless steel.
2. The rolling bearing of claim 1, wherein the surface hardness (Hv) of the
finished raceway track on at least one race selected from the group
consisting of said outer race and said inner race is not less than 600.
3. A rolling bearing comprising races composed of an outer race and an
inner race and rolling elements which are provided between the outer race
and the inner race such that the rolling elements rotate freely, wherein
at least said inner race is made of a titanium alloy, and wherein a
surface of a finished raceway track on at least one race selected from the
group consisting of said outer race and said inner race comprises a
mixture of .alpha. phase texture and .beta. phase texture, the proportion
of said .beta. phase in said mixture being from 30 to 80 vol %.
4. The rolling bearing of claim 3, wherein the surface hardness (Hv) of the
finished raceway track on at least one race selected from the group
consisting of said outer race and said inner race is not less than 600.
Description
FIELD OF THE INVENTION
The present invention relates to a rolling bearing, and, more particularly,
a rolling bearing which is used which is used under a special environment,
for example, under an environment requiring corrosion resistance to water
content, sea water and chemicals, e.g., in a food machine, a semiconductor
producing apparatus and a chemical fiber producing machine, or in a tool
machine which operates at a high rotary speed.
BACKGROUND OF THE INVENTION
As a bearing which must be corrosion-resistant there has been heretofore
used relatively often a sliding bearing made of a material having an
excellent corrosion resistance. In recent years, rolling bearings have
been used more and more from the standpoint of torque reduction that
prevents dynamic loss or eliminates the necessity of maintenance and
improvement of product quality.
As the material for such rolling bearings there is mostly used a low-alloy
steel such as two kinds of high carbon chromium bearing steels (SUJ2) and
case hardening steel (SCR420). However, rolling bearings are used in
various working conditions. Thus, if such a rolling bearing made of a
low-alloy steel is used under environmental conditions which can be
contaminated by water content or sea water, the contamination by even a
slight amount of water content or sea water corrodes the bearing portion
thereof corrodes with rust that disables the rolling bearing from working.
Thus, martensite stainless steel having an excellent corrosion resistance
and a high chromium content (e.g., SUS440C) is used under such
environmental conditions.
However, a rolling bearing comprising races and rolling elements both of
which are made of martensite-based stainless steel (hereinafter simply
referred to as "stainless steel") can exhibit an insufficient corrosion
resistance in some working atmospheres. In this case, corrosion occurs
with chromium-deficient layer in the vicinity of coarse eutectic carbide
as a starting point to reduce precision such as surface smoothness,
possibly making it impossible to secure the desired bearing life. In
particular, a rolling bearing adapted for use in semiconductor producing
apparatus, etc. is subject to attack by a corrosive gas or chemical that
can corrode stainless steel. Thus, it is required that such a rolling
bearing comprise a material having a better corrosion resistance than
stainless steel.
From this standpoint of view, as a bearing material constituting a rolling
bearing adapted for use in corrosive working atmospheres there has
heretofore been used a ceramic material such as silicon nitride (Si.sub.3
N.sub.4) (hereinafter referred to as "first conventional technique").
In the machine tool industry, on the other hand, the recent trend is for
more machines to operate at higher rotary speed. To this end, it is
required for the rolling bearing for supporting the rotary portion of
machine tools to have higher precision and withstand severer working
conditions. When a machine tool operates at a raised rotary speed, the
so-called bearing clearance is reduced, causing further rolling friction
that adds to heat generation. As a result, the temperature of the bearing
rises.
The rise in the heat generation due to rolling friction is considered to be
attributed to the rise in the centrifugal force applied to the rolling
elements. In order to lessen the centrifugal force and hence lower the
temperature of the rolling elements, a rolling bearing comprising rolling
elements made of ceramic material, which exhibit a small density (specific
gravity), rather than low-alloy steel has heretofore been put into
practical use. However, with the recent trend for more machine tools to
operate at even higher rotary speed, mere reduction of the weight of the
rolling elements cannot prevent the rise in the bearing temperature.
By the way, the heat generated in the outer race during high speed rotation
normally is radiated to the exterior through the housing. Since the heat
generated in the inner race can be difficultly radiated from the rotary
axis, the temperature of the inner race is higher than that of the outer
race. Thus, if the outer race and the inner race are formed by the same
material, and the temperature of the inner race is raised by heat
generation, the inner race undergoes a great thermal expansion that
reduces the bearing clearance from the initial value. The resulting
preload is excessive, accelerating the heat generation. This phenomenon
occurs in a vicious circle. Eventually, the bearing undergoes seizing that
can lead to the destruction of the bearing.
From this standpoint of view, a rolling bearing has been proposed
comprising an inner race formed by a material having a smaller linear
expansion coefficient than the outer race material (see JP-B-7-30788 (The
term "JP-B" as used herein means an "examined Japanese patent
publication")) (hereinafter referred to as "second conventional
technique"). In accordance with the foregoing second conventional
technique, the inner race is formed by a material having a smaller linear
expansion coefficient than the outer race material. For example, the outer
race may be formed by a high carbon chromium bearing steel (SUJ2) while
the inner race may be formed by a stainless steel (SUS440C) or ceramic
material. In this arrangement, even if the temperature of the inner race
is higher than that of the outer race, the expansion of the inner race
caused by the temperature difference between the inner race and the outer
race can be inhibited. As a result, the variation of preload accompanying
the change in the bearing clearance is reduced, making it possible to
prevent the bearing from seizing.
A titanium alloy has a lighter weight and a higher strength than a steel
material and a very excellent corrosion resistance among metallic
materials and thus is expected to be a bearing material for use in special
corrosive atmospheres such as those contaminated by water content, sea
water, chemical, etc.
In a rolling bearing, however, a very great face pressure is applied to the
portion at which the races and the rolling elements come in contact with
each other. Thus, it is required for a rolling bearing to exhibit a high
surface hardness. However, a titanium alloy which has been merely
subjected to ordinary heat treatment such as solution treatment and aging
cannot be provided with a desired surface hardness.
From this standpoint of view, a technique for enhancing the surface
hardness of a titanium alloy by a predetermined surface treatment has been
proposed (JP-B-61-2747) (hereinafter referred to as "third conventional
technique").
In the foregoing third conventional technique, a titanium alloy is
subjected to gaseous nitriding or carburizing so that penetrating elements
such as C, N and O are diffused in the form of solid solution therein,
thereby securing the surface hardness required for the races.
In the foregoing first conventional technique, a ceramic material is used
as bearing material. Thus, the bearing exhibits an extremely good
corrosion resistance as compared with stainless steel. However, the first
conventional technique is disadvantageous in that a ceramic material is
inferior to stainless steel in strength or toughness and thus cannot be
used without any trouble in atmospheres subject to great load. In
particular, the use of ceramic material as the race material is
undesirable from the standpoint of reliability of bearing.
Further, a ceramic material is remarkably inferior to metallic material in
formability and grindability. Thus, if all the essential parts of a
bearing are formed by a ceramic material, it disadvantageously adds to the
production cost.
Moreover, a ceramic material has an extremely smaller linear expansion
coefficient than a metallic material. Thus, the foregoing conventional
technique has some disadvantages. For example, if the outer race is formed
by the foregoing high carbon chromium steel (SUJ2) and the inner race is
formed by a ceramic material, the difference in thermal expansion between
the metallic rotary axis and the inner race made of ceramic material
becomes too great when the temperature rises to relax the thermal
expansion of the rotary axis, possibly cracking the inner race made of
ceramic material and hence causing the destruction of the bearing.
On the other hand, if the outer race is formed by a high carbon chromium
bearing steel (SUJ2) and the inner race is formed by a stainless steel
(SUS440C), the change in the bearing clearance caused by the temperature
rise can be minimized because the linear expansion coefficient of
stainless steel is as small as 80% of that of high carbon chromium bearing
steel. Further, since a stainless steel is a metallic material, the inner
race made of stainless steel is considered to be insusceptible to cracking
due to the difference in thermal expansion between the rotary axis and the
inner race unlike the inner race made of ceramic material.
However, since the stainless steel used as inner race material has a higher
density (higher specific gravity) than the ceramic material, the rise in
the centrifugal force applied to the inner race cannot be neglected. In
other words, since centrifugal force increases in proportion to mass and
speed, the inner race expands due to the centrifugal force produced by
rotation as the rotary speed increases. As a result, the bearing clearance
is reduced, accelerating the heat generation.
The foregoing third conventional technique is disadvantageous in that the
resulting surface hardness and depth of hardening differ greatly with the
kind of penetrating elements to be incorporated in the form of solid
solution by surface treatment. Further, some titanium alloys used have too
low a strength in the core to fulfill a sufficient function as bearing.
In accordance with the third conventional technique, the surface hardness
of the titanium alloy can be enhanced by diffusing penetrating elements in
the titanium alloy in the form of solid solution. However, these
penetrating elements can embrittle the titanium alloy, making it
impossible to obtain a desired bearing life.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a rolling
bearing excellent in corrosion resistance, toughness and high rotary speed
operation.
The foregoing and other objects of the present invention will become more
apparent from the following detailed description and examples.
The objects are achieved by the following embodiments mainly.
(1) A rolling bearing comprising races composed of an outer race and an
inner race and rolling elements which are provided between the outer race
and the inner race such that the rolling elements rotate freely, wherein
at least the inner race is made of a titanium alloy and the rolling
elements are made of a corrosion-resistant material.
(2) The rolling bearing of item (1), wherein the titanium alloy is selected
from the group consisting of .beta. type titanium alloy and
(.alpha.+.beta.) type titanium alloy and the corrosion-resistant material
is selected from the group consisting of ceramics and martensite stainless
steel.
(3) The rolling bearing of item (1), wherein the surface hardness (Hv) of
the finished raceway track on at least one race selected from the group
consisting of the outer race and the inner race is not less than 600.
(4) The rolling bearing of item (1), wherein the surface of the finished
raceway track on the at least one race comprises a mixture of a phase
texture and .beta. phase texture, the proportion of the .beta. phase in
the mixture being from 30 to 80 vol %.
(5) A method for producing a rolling bearing, which comprises preparing at
least one race selected from the group consisting of an outer race and an
inner race according to a method which comprises steps of:
(a) selecting at least one from the group consisting of .beta. type
titanium alloy and (.alpha.+.beta.) type titanium alloy as a race
material;
(b) heating and keeping said race material at the temperature falling
within the range of .beta. phase temperature of not lower than .beta.
phase transition point (.beta.-phase transus) to effect solution treatment
such that the phase of the texture of said race material is converted to
.beta. phase;
(c) rapidly cooling said race material so that the texture of said race
material normally stays in .beta. single phase;
(d) subjecting said race material to plastic working (cold working) so that
it is shaped as desired and given work strain, which enables formation of
nuclei of .alpha. phase which is harder than .beta. phase and the .alpha.
phase to be finely deposited in .beta. phase;
(e) subjecting said race material to aging at a predetermined temperature
lower than .beta. phase transition point, whereby nuclei of .alpha. phase
are formed and grown and the .alpha. phase is finely deposited in .beta.
phase; and then
(f) machining said race material to a race.
(6) The method of item (5), wherein the percent plastic working at the step
(d) is not less than 20%.
(7) The method of item (5), wherein the percent plastic working is from 5
to 30% and the surface of the raceway track is subjected to shot peening
before aging.
(8) The method of item (7), wherein shot peening is effected after aging.
BRIEF DESCRIPTION OF THE DRAWINGS
By way of example and to make the description more clear, reference is made
to the accompanying drawings in which:
FIG. 1 is a chart illustrating a second embodiment of the method for the
production of the bearing material according to the embodiment of the
present invention;
FIG. 2 is a chart illustrating a third embodiment of the method for the
production of the bearing material according to the embodiment of the
present invention;
FIG. 3 is a chart illustrating a modification of the third embodiment of
the method for the production of the bearing material according to the
embodiment of the present invention;
FIG. 4 is a diagram illustrating the inner structure of a submerged thrust
bearing life testing machine for use in the submerged life test;
FIG. 5 is a sectional view illustrating a high speed rotary testing machine
for use in the high speed rotary test;
FIG. 6 is a characteristic curve illustrating the relationship between
percent cold working .eta. and hardness Hv after aging in the fourth group
of examples;
FIG. 7 is a characteristic curve illustrating the relationship between
percent cold working .eta. and hardness Hv after aging in the fifth group
of examples;
FIG. 8 is a characteristic curve illustrating the relationship between
aging time and residual .beta. phase content and hardness Hv after aging
in the sixth group of examples; and
FIG. 9 is a characteristic curve illustrating the relationship between
residual .beta. phase and submerged life L.sub.10 in the sixth group of
examples, wherein the reference numeral 3 indicates an inner race, the
reference numeral 4 indicates an outer race, the reference numeral 5
indicates rolling elements, the reference numeral 12 indicates a outer
race, the reference numeral 13 indicates an inner race, and the reference
numeral 14 indicates rolling elements.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be further described hereinafter.
The inventors made extensive studies of rolling bearing having an excellent
corrosion resistance. As a result, it was found that the use of a titanium
alloy having a higher toughness than ceramics as a race material makes it
possible to drastically improve corrosion resistance as compared with the
use of stainless steel.
It was also found that an inner race formed by such a titanium alloy, which
has a lighter weight and a smaller linear expansion coefficient than
stainless steel, shows a smaller temperature rise during high speed
operation than that formed by stainless steel, making it possible to avoid
the reduction of clearance and hence inhibit the rise in heat generation.
The present invention has been worked out on the basis of these knowledges.
As the first feature, the rolling bearing according to the present
invention comprises an outer race and an inner race and rolling elements
rotatably provided between the outer race and the inner race, wherein at
least the inner race is made of a titanium alloy and the rolling elements
are made of a corrosion-resistant material.
The inventors obtained a knowledge that among titanium alloys having an
excellent corrosion resistance a .beta. type titanium alloy exhibits a
high strength and an excellent cold-workability in the form of solid
solution and then made extensive studies. As a result, it was found that
the use of a .beta. type titanium alloy cold-worked at a percent cold
working (percent plastic working) of not less than 20% as a bearing
material makes it possible to provide a race having a Rockwell hardness
HRC (hereinafter simply referred as "HRC") of not less than 57 through a
short aging.
Thus, as the second feature, in the rolling bearing according to the
present invention, at least one of the inner race and outer race is formed
by a .beta. type titanium alloy cold-worked at a percent cold working of
not less than 20%.
When a titanium alloy which has been cold-worked by not less than 20% is
subjected to aging, a rolling bearing having a desired surface hardness as
defined above can be obtained. However, the resulting .beta. type titanium
alloy tends to have a hardened texture as a whole. In particular, if the
percent cold working is predetermined high, the .beta. type titanium alloy
hardens more than necessary even in its core and to thereby exhibit a
reduced toughness. Accordingly, from the point of view of obtaining good
toughness, it appears to be preferred that the titanium alloy be not
subjected to cold working or, if any, be subjected to cold working at a
low percent working to obtain a good toughness.
If a steel material such as stainless steel is used as a race material, it
is subjected to heat treatment such as hardening and tempering and then to
shot peening to have an enhanced surface hardness. In other words, when
subjected to shot peening, the stainless steel material undergoes
transformation of residual austenite to martensite, producing stress that
gives a huge strain energy to the surface layer of the race. The
work-hardening makes it possible to enhance the surface hardness of the
race.
However, the inventors' studies made it obvious that if a titanium alloy
which has been subjected to heat treatment is subjected to shot peening
alone, the amount and depth of work strain thus provided are restricted,
making it difficult to obtain a desired surface hardness required for
rolling bearing.
Paying their attention to the rise in surface hardness by shot peening, the
inventors made further extensive studies. As a result, it was found that a
titanium alloy which has been cold-worked at a percent working of from 5
to 20% can be subjected to shot peening to obtain a rolling bearing having
a good toughness as well as a high surface hardness.
As the other feature, in the rolling bearing according to the present
invention, at least one of the inner race and outer race is formed by a
.beta. type titanium alloy obtained by cold working at a percent working
of from 5 to 20%, followed by shot peening.
In the foregoing aspect of the present invention, if a titanium alloy which
has been cold-worked is subjected to shot peening followed by aging, a
rolling bearing having a Vickers surface hardness Hv (hereinafter simply
referred to as "Hv") of not less than 600 (corresponding to HRC of about
57) can be obtained. In order to improve fatigue resistance, the titanium
alloy which has been aged is preferably again subjected to shot peening.
During their study of the life of a race made of a titanium alloy, the
inventors found that the bearing shows a shorter life when the lubricant
is contaminated by foreign matters than when the lubricant is free of
foreign matters similarly to the case where the race is made of a steel
material such as stainless steel.
For the conventional rolling bearings made of steel material, a technique
for improving the life of bearing by optimizing the carbon content and
residual austenite, optionally carbon nitride content, in the surface
layer of the bearing is proposed in JP-B-7-88851. In the known technique,
by optimizing and restricting the content of carbon, residual austenite
and carbon nitride in the material to a specific range, the concentration
of stress on the edge portion of impression produced by foreign matters
can be relaxed, inhibiting the generation of cracks. As a result, the life
of the bearing can be improved.
The optimum relationship between the amount of residual austenite and the
surface hardness is found by adjusting and optimizing the average grain
diameter of carbide or carbon nitride and a technique for prolonging the
life of bearing based on the relationship is proposed in JP-B-8-26446.
In other words, the foregoing known techniques (JP-B-788851, JP-B-8-26446)
contemplate optimizing the amount of soft austenite to improve the life of
bearing when the lubricant is contaminated by foreign matters.
Accordingly, it is considered that even a .beta. type titanium alloy can
provide a bearing which can operate over a prolonged life even when the
lubricant is contaminated by foreign matters if the volumetric proportion
of residual .beta. phase being a soft phase is optimized.
The inventors made extensive studies from such a standpoint of view. As a
result, it was found that if the volumetric proportion of residual .beta.
phase in soft phase in the texture of .beta. type titanium alloy is
optimized, a bearing can be obtained which can operate over a desired life
even when the lubricant is contaminated by foreign matters.
As the fourth feature, in the rolling bearing according to the present
invention, at least one of the inner race and outer race is formed by a
.beta. type titanium alloy obtained by cold working at a percent working
of not less than 20%, and the volumetric proportion of residual .beta.
phase in the .beta. type titanium alloy is from 30 to 80%.
In the foregoing aspect of the present invention, if the percent cold
working is predetermined to 5 to 20% on condition that a titanium alloy
which has been cold-worked is subjected to shot peening, a rolling bearing
which satisfies both the two requirements for toughness and surface
hardness can be obtained.
FIRST EMBODIMENT
In the rolling bearing according to the first embodiment of the present
invention, at least the inner race is formed by a titanium alloy, and the
rolling elements are formed by a corrosion-resistant material such as
ceramics.
The reason why the rolling bearing and the rolling elements are formed by
these materials will be described hereinafter.
(1) Races
The terminology "a race" as used hereinafter inclusively means an inner
race and an outer race.
A race formed by a titanium alloy exhibits a drastically improved corrosion
resistance as compared with that formed by stainless steel.
The corrosion resistance of titanium is attributed to the formation of a
stable passive film on the surface thereof similarly to stainless steel.
The passive film of titanium is known to be TiO.sub.2 (or Ti.sub.2
O.sub.3) (see Goro Ito, "Fushoku kagaku to boshoku gijutsu (Corrosion
science and corrosion prevention technique)", revised edition, page 282,
Corona Co., Ltd., 1979). Thus, the excellent corrosion resistance of
titanium is attributed to properties inherent to the passive film of
titanium.
In other words, TiO.sub.2, which is the passive film of titanium, exhibits
a high oxygen overvoltage. As the potential applied to titanium rises, the
animation proceeds. The resulting passive film exhibits an excellent
corrosion resistance even in a high temperature high concentration
oxidizing atmosphere such as high temperature high concentration nitric
acid. Unlike stainless steel, titanium does not undergo corrosion due to
overpassivation.
On the other hand, TiO.sub.2 corrodes in a nonoxidizing atmosphere such as
hydrochloric acid and sulfuric acid easily but less easily than stainless
steel. Further, TiO.sub.2 requires a low passivation potential for forming
a passive film. Therefore, a titanium alloy can be easily passivated
merely by dipping it in a corrosive solution comprising an extremely small
amount of an oxidizing agent incorporated therein. Accordingly, a titanium
alloy can be corrosion-resistant even in a nonoxidizing atmosphere such as
hydrochloric acid and sulfuric acid.
Further, the passive film is tough and does not break even when attacked by
chloride ion. Thus, the passive film is little liable to erosion, void
corrosion, stress corrosion cracking, etc., which are remarkable in
stainless steel. Accordingly, the passive film exhibits an extremely
excellent corrosion resistance against sea water. As a result, a rolling
bearing formed by a titanium alloy cannot be disabled even when sea water
enters thereinto.
Moreover, a titanium alloy also exhibits an excellent corrosion resistance
against many organic acids and is not liable to deterioration of
cold-workability or deterioration by impure elements.
Thus, a titanium alloy exhibits an extremely excellent corrosion resistance
as compared with stainless steel.
The comparison of titanium alloy with ceramics material such as Si.sub.3
N.sub.4 in corrosion resistance shows that a titanium alloy undergoes
so-called overall corrosion against some alkaline solutions such as NaOH
and KOH solutions and thus cannot be used in such an alkaline atmosphere
but exhibits corrosion resistance equal to ceramics in special atmospheres
other than the alkaline atmosphere.
A ceramics material exhibits a low toughness and thus is not suitable for
use under working conditions subject to great impact load while a titanium
alloy exhibits a toughness about three times that of Si.sub.3 N.sub.4. In
other words, a titanium alloy exhibits a toughness equal to stainless
steel. Thus, if a titanium alloy is used as a race., it is extremely
unlikely that the bearing can break as compared with the case where a
ceramics material is used.
Further, ceramics cannot be subjected to plastic working as metallic
materials. Therefore, in order to produce a race from ceramics, ceramics
must be subjected to a continuous complicated production method which
comprises compressing a powdered ceramics into a ring, sintering the
material, subjecting the material to HIP (hot isostatic pressing) so that
it is densed, and then grinding the material. Thus, ceramics materials
exhibit a poor productivity as compared with metallic materials. Further,
a large-sized race can hardly be produced from a ceramics material.
Moreover, ceramics materials exhibit a remarkably deteriorated
grindability as compared with metallic materials, thereby increasing the
production cost.
On the other hand, a titanium alloy exhibits a deteriorated workability as
compared with a steel material such as stainless steel but a sufficient
plastic deformability. A titanium alloy exhibits an excellent grindability
as compared with ceramics. Further, working facilities for steel material
can be used for titanium alloy. Therefore, existing facilities can be
used, eliminating the necessity of equipment investment. The production
cost can be reduced.
A titanium alloy is a nonmagnetic material. Thus, even if a titanium alloy
is used in a magnetic atmosphere such as semiconductor producing apparatus
and superconduction-related apparatus, disturbance in the magnetic field
can be avoided. Further, the rise or variation in the rotary torque of the
bearing due to magnetic field can be inhibited.
The inhibition of the rise or variation in the rotary torque is more
remarkable when the rolling elements are formed by a nonmagnetic ceramics.
On the other hand, in order to avoid the rise in the bearing temperature
even in a tool machine which operates at a high rotary speed, it is
effective to form the races, particularly inner race, by a titanium alloy.
As previously mentioned, the rise in the bearing temperature developed when
the bearing rotates at a high speed is attributed to the reduction of
bearing clearance accompanying the high speed rotation. The reduction of
bearing clearance is attributed not only to the thermal expansion due to
the difference in temperature between the inner race and the outer race
but also to the expansion of the inner race due to the centrifugal force
caused by the rotation of the rotary axis.
Accordingly, in order to inhibit the rise in the bearing temperature
accompanying the high speed rotation, it is necessary that a material
having a small linear expansion coefficient be selected to inhibit the
thermal expansion. In order to reduce the centrifugal force, it is
necessary that a material having a small density be selected.
The comparison of Ti-6Al-4V alloy as a titanium alloy with SUS440C as a
stainless steel shows that the linear expansion coefficient of Ti-6Al-4V
alloy is as small as 80% of that of SUS440C. Therefore, if Ti-6Al-4V alloy
is used as an inner race material, the reduction in the bearing clearance
accompanying the difference in temperature between the inner race and the
outer race can be drastically reduced as compared with the use of
stainless steel.
Further, the density of Ti-6Al-4V alloy is as small as about 60% of that of
SUS440C. As a result, Ti-6Al-4V alloy gives a lower centrifugal force than
stainless steel. Thus, the inner race formed by Ti-6Al-4V alloy expands
less than that formed by stainless steel.
As mentioned above, by using a titanium alloy as an inner race, the
reduction in the bearing clearance during high speed rotation can be
avoided, thereby inhibiting the rise in friction. As a result, the rise in
the bearing temperature can be inhibited.
When the race and the rolling elements come in contact with each other
under a predetermined load, the contact portion undergoes elastic
deformation to form a contact ellipse the size of which depends on the
Young's modulus of the race and the rolling elements.
Ceramics exhibit a greater Young's modulus than metallic materials and thus
undergo little elastic deformation. Accordingly, the race receives a
higher face pressure when the rolling elements are formed by ceramics than
when the rolling elements are formed by a metallic material. On the other
hand, a titanium alloy exhibits a Young's modulus as small as about half
that of stainless steel. Accordingly, the contact ellipse is larger when
the race is formed by a titanium alloy than when the race is formed by a
stainless steel. Thus, the contact portion receives a lower face pressure
when the race is formed by a titanium alloy than when the race is formed
by a stainless steel. Therefore, the use of a titanium alloy as a race
makes it possible to relax the rise in the contact face pressure which can
occur when rolling elements made of ceramics is used and improve the
rolling fatigue life of the bearing.
As the titanium alloy to be used for race there may be used
(.alpha.+.beta.) type titanium alloy such as Ti-6Al-4V, Ti-3Al-2.5V and
Ti-6Al-2Sn-4Zr-6Mo or .beta. type titanium alloy such as Ti15Mo-5Zr,
Ti-15Mo-5Zr-3Al, Ti-15V-3Sn-3Al-3Cr, Ti-10V-2Fe-3Al, Ti-3Al-8V-6 Cr-4Zr
and Ti-22V-3Al, which can be subjected to heat treatment to have a high
strength and a high toughness.
Preferred among the titanium alloys listed above are .beta. type titanium
alloys, which exhibit an excellent cold-workability, taking into account
workability. Particularly preferred among these .beta. type titanium
alloys are Ti-15Mo titanium alloys such as Ti-15Mo-5Zr and
Ti-15Mo-5Zr-3Al, which are particularly excellent in corrosion resistance.
(.alpha.+.beta.) type titanium alloys have a great content of alloying
elements having a smaller density than Ti. Thus, (.alpha.+.beta.) type
titanium alloys, which have a small mass, are preferably used in terms of
reduction of centrifugal force.
In order to secure the bearing strength, the titanium alloy needs to be
subjected to heat treatment so that it is reinforced as a (.alpha.+.beta.)
two-phase texture.
Pure titanium and .alpha. type titanium alloy such as Ti-0.3Mo-0.8Ni have
an .alpha. single phase microstructure and hence a lower strength than the
foregoing (.alpha.+.beta.) type titanium alloys or .beta. type titanium
alloys and thus cannot used as race materials.
It is said that the surface hardness HRC of the race needs to be not less
than 57 to provide an endurable bearing. However, if the foregoing
titanium alloy is used as a race, even if the material has been hardened
by aging after solution treatment, the resulting surface hardness is as
small as about 40 to 45, making it impossible to provide a surface
hardness required for bearing. Further, the resulting bearing exhibits a
poor seizing resistance and thus is liable to adhesive abrasion.
The foregoing titanium alloy is preferably subjected to heat treatment such
as atmospheric oxidation, gaseous nitriding, boriding, wet plating, TiC or
TiN coating by CVD method or PVD method and ion injection to obtain a
desired surface hardness HRC. Taking into account the convenience of
treatment, atmospheric oxidation or gaseous nitriding is desirable.
In the present embodiment, at least the inner race is formed by a titanium
alloy. In a preferred embodiment, both the inner race and the outer race
are formed by a titanium alloy to provide a better corrosion resistance in
a working atmosphere such as food machine, semiconductor producing
apparatus and chemical fiber producing machine which is liable to be
contaminated by a corrosive material such as water content, sea water and
chemicals. In a machine tool or other machines which operate at a high
rotary speed, it is important to inhibit the rise in the inner race
temperature. Therefore, the inner race needs to be formed by a titanium
alloy, but the outer race is preferably formed by a steel material such as
SUJ2 and stainless steel, which exhibits a greater linear expansion
coefficient than the titanium alloy constituting the inner race.
(2) Rolling Elements
The reason why the rolling elements are formed by ceramics in the present
embodiment will be described hereinafter.
Ceramics are insulating materials. Rolling elements formed by ceramics is
not liable to so-called galvanic corrosion even when it comes in contact
with a race formed by a titanium alloy and thus is extremely excellent in
corrosion resistance as compared with that formed by a metallic material.
Ceramics are also nonmagnetic materials. Thus, rolling elements formed by
ceramics causes no variation of rotary torque of bearing even when used in
a magnetic field. Accordingly, ceramics are suitable for use in a special
working atmosphere subject to magnetic field such as semiconductor
producing apparatus and superconduction-related apparatus.
Further, ceramics have a smaller density than stainless steel. The
comparison of Si.sub.3 N.sub.4 as ceramics with SUS440C as stainless steel
shows that the density of Si.sub.3 N.sub.4 is about 40% of that of
SUS440C. Accordingly, the use of ceramics, which have a smaller density
than stainless steel, makes it possible to provide rolling elements having
a lighter weight. When the rolling bearing operates at a high rotary
speed, such rolling elements give a reduced centrifugal force that applies
a reduced load to the outer race, making it possible to inhibit the
deterioration of durability.
In other words, when a rolling bearing operates at a high rotary speed, the
high speed rotation is accompanied by the rise in centrifugal force that
causes the rolling elements to apply nonneglible load to the outer race.
Thus, the contact load of the rolling elements on the outer race is
raised, reducing the life of bearing or raising the amount of heat
generated by friction. Further, since the centrifugal force of the rolling
elements are proportional to the mass of the rolling elements as well
known, the greater the mass of the rolling elements are, the greater is
the foregoing contact load.
Thus, in the present embodiment, the use of ceramics as rolling element
material provides rolling elements having a reduced weight that inhibits
the generation of heat by friction and hence the reduction of the life of
bearing.
In a rolling bearing having a contact angle such as angular contact ball
bearing, the rolling elements are acted upon by gyroscopic moment. When
the gyroscopic moment becomes greater than the frictional force at the
portion where the rolling elements come in contact with the race, a
violent revolutionary slip called skidding occurs to cause further
friction. The reduction of the weight of the rolling elements also makes
it possible to reduce the gyroscopic moment.
Further, rolling elements formed by the same titanium alloy having an
excellent corrosion resistance as used for the race exhibit a strong
adhesion and thus is liable to seizing or galling. On the contrary,
rolling elements formed by ceramics, which differ from the material of the
race, exhibit improved seizing resistance and galling resistance. In
particular, a titanium alloy is an active metal and thus exhibits a
deteriorated seizing resistance. Accordingly, the use of ceramics as
rolling element material makes it possible to improve the seizing
resistance of the titanium alloy used as race.
As the ceramics to be used as rolling element material there may be used
SiAlON, zirconia (ZrO.sub.2), silicon carbide (SiC), alumina (Al.sub.2
O.sub.3) or the like besides Si.sub.3 N.sub.4. Si.sub.3 N4 exhibits a
small density, a low linear expansion coefficient, a high thermal impact
resistance and excellent flexural strength and fracture toughness and thus
can be used as rolling elements for use under high speed rotary
conditions.
The present invention is not limited to the present embodiment. With
respect to the bearing for use in a corrosive working atmosphere, the
rolling elements are preferably formed by a stainless steel depending on
the application.
In this case, the rolling elements are formed by a stainless steel, which
differ from the material of the race, i.e., titanium alloy as in the case
where the rolling elements are formed by ceramics. When the bearing
rotates, the different kinds of metals come in contact with each other.
In general, when different kinds of metals come in contact with each other
in a solution, galvanic corrosion occurs to accelerate the corrosion of
the metal which is electronegatively greater than the other. Accordingly,
when rolling elements made of stainless steel, which is electronegatively
greater than titanium alloy, come in contact with a race made of titanium
alloy, the rolling elements corrode remarkably, possibly causing a drastic
reduction of the bearing life.
Stainless steel is electronegatively greater than titanium alloy in the
order of corrosion tendency in sea water. However, the two metals have an
extremely small potential difference (see "Titan Kako Gijutsu (Titanium
Processing Technique)", compiled by Japan Titanium Society, page 208
(published by Nikkan Kogyo Shinbunsha, 1992). Thus, little or no galvanic
corrosion occurs even when a titanium alloy and a stainless steel come in
contact with each other in sea water.
Accordingly, as the rolling element material there may be used a
general-purpose stainless steel in some cases. In other words, in some
cases, the use of stainless steel as rolling element material rather than
expensive ceramics makes it possible to maintain sufficient corrosion
resistance and hence reduce the production cost. Further, the use of
stainless steel as rolling element material also makes it possible to
reduce the contact face pressure as compared with ceramics material which
is little liable to elastic deformation.
SECOND EMBODIMENT
In the rolling bearing according to the second embodiment of the present
invention, at least one of the inner race and outer race is formed by a
.beta. type titanium alloy and the percent cold working of the race is
predetermined to not less than 20%.
Among the titanium alloys having an excellent corrosion resistance, a
.beta. type titanium alloy exhibits a high strength and an excellent
cold-workability in the form of solid solution. In other words, a .beta.
type titanium alloy which has been subjected to solution treatment at a
predetermined temperature can be rapidly cooled to obtain a soft .beta.
single phase having a body-centered cubic lattice (bcc) structure at room
temperature. Among materials belonging to .beta. type titanium alloy,
there is a reinforcible material having a percent cold working .eta. of
not less than 90% as represented by the following equation (1). The use of
such a material makes it possible to omit the grinding step.
.eta.={(1.sub.0 -1)/1.sub.0 }.times.100 (1)
wherein 1.sub.0 represents the height of the material before cold working;
and 1 represents the height of the material after cold working.
In other words, a titanium alloy exhibits an excellent corrosion resistance
but a small thermal conductivity and thus generates heat at the area where
it comes in contact with the grinding tool during grinding that gives a
great stress to the cutting edge. Thus, a titanium alloy is
disadvantageous in that it exhibits a deteriorated grindability. In the
second embodiment of the present invention, .beta. type titanium alloy,
which exhibits an excellent cold-workability, is used. The .beta. type
titanium alloy is subjected to solution treatment to give a soft .beta.
single phase which is then subjected to cold working. This cold working
causes the production of a large amount of lattice defects that cause
dislocation. Thus, hard .alpha. phase is uniformly and finely deposited in
.beta. crystalline grains. In this manner, both the surface hardness HRC
and the strength of the material can be enhanced, making it possible to
enhance the durability of the rolling bearing itself.
In other words, it is a common practice that the bearing material which has
been subjected to solution treatment is subjected to aging for hardening.
However, if the bearing material which has been subjected to solution
treatment is not subjected to cold working before aging, .alpha. phase is
deposited preferentially at the grain boundary in layer during aging but
less in .beta. crystalline grains, providing an extremely nonuniform aged
texture.
On the contrary, if the bearing material which has been subjected to
solution treatment is subjected to cold working before aging, the cold
working (plastic working) causes a large amount of dislocation to be
introduced into .beta. crystalline grains, and the dislocation becomes a
nucleus production ground for deposition of .alpha. phase. Thus, hard
.alpha. phase is uniformly and finely deposited in soft .beta. crystalline
grains, increasing the surface hardness of the material.
In other words, a .beta. type titanium alloy obtained by aging a titanium
alloy which has been subjected to solution treatment free from cold
working has a surface hardness HRC of about from 40 to 48. On the
contrary, a titanium alloy obtained by subjecting a solution-treated
titanium alloy to cold working followed by aging can be provided with a
surface hardness HRC of not less than 57 and hence a raised strength that
improves the life of the rolling bearing.
The solution treatment temperature, percent cold working .eta. and aging
time T will be described hereinafter.
(1) Solution Treatment Temperature
If solution treatment is effected at a temperature of not higher than the
critical temperature at which .beta. transition, i.e., .beta. phase is
transformed to (.alpha.+.beta.) phase, initial .alpha. phase is deposited,
causing a remarkable deterioration of workability. Accordingly, the
solution treatment temperature needs to be not lower than .beta.
transition. On the contrary, if solution treatment is effected at an
excessively high temperature, the resulting .beta. crystalline grains are
remarkably coarse, causing a strength drop. Thus, in the present
embodiment, the solution treatment temperature is predetermined to a range
of from .beta. transition to (.alpha.+150.degree. C.).
(2) Percent Cold Working .eta.
A titanium alloy obtained by subjecting a solution-treated titanium alloy
to cold working before aging exhibits enhanced surface hardness HRC and
strength. As described later, such a titanium alloy which has been
subjected to cold working can be aged in a reduced time. However, the
density of dislocation introduced by cold working varies, affecting the
surface hardness HRC or strength. In other words, if the percent cold
working .eta. is predetermined to not more than 20%, the resulting
dislocation is nonuniform, causing .alpha. phase to be deposited
preferentially at the grain boundary. Further, when .alpha. phase is
deposited in layer at the grain boundary, break can easily occur at the
interface of .beta. crystalline grain with .alpha. phase, causing a
strength drop.
On the contrary, if the percent cold working .eta. is not less than 20%,
dislocation is uniformly introduced into crystalline grains. Thus, .alpha.
phase is uniformly and finely deposited in .beta. crystalline grains with
the foregoing dislocation as a nucleus production ground during aging,
enhancing the surface hardness HRC and strength.
It is considered that the degree of reinforcing by cold working follows
n-order hardening rule represented by the equation (2):
.sigma.=AE.sup.n (2)
wherein .sigma. represents true stress; E represents true strain; A
represents reinforcement coefficient; and n represents work-hardening
index. A .beta. type titanium alloy exhibits a smaller work-hardening
index than steel material and thus is akin to completely plastic material.
Thus, the percent cold working .eta. can be raised without any problem. In
particular, when the percent cold working .eta. is within the range of not
less than 30%, a bearing material having a stabilized hardness can be
obtained. Accordingly, cold working may be effected at a percent cold
working .eta. of not less than 20% to obtain a predetermined height.
From these standpoints of view, the percent cold working .eta. is
predetermined to not less than 20%, preferably not less than 30%, in the
present embodiment.
(3) Aging Time T
As mentioned above, the dislocation introduced during cold working becomes
a nuclear production ground which accelerates the deposition of .alpha.
phase in .beta. crystalline grains. As a result, the time required until
overaging is reached can be reduced, making it possible to drastically
reduce the aging time T. However, if aging is effected over an excessively
prolonged period of time, averaging occurs, causing hard .alpha. phase to
grow coarsely. Thus, the material softens, causing a drop of surface
hardness HRC and hence a reduction of the bearing life. Further, if the
aging time T is predetermined excessively long, an intermetallic compound
is deposited as a final stable phase, remarkably embrittling the bearing
material. As a result, the surface hardness and submerged life of the
bearing can be reduced. From these standpoints of view, the aging time T
is preferably predetermined to 5 to 10 hours in the present embodiment.
FIG. 1 is a chart illustrating the method for the production of the bearing
material according to the embodiment of the present invention.
In other words, a .beta. type titanium alloy is subjected to solution
treatment at a temperature (.beta. transition to (.beta. transition
+150.degree. C.), e.g., 800.degree. C. to 1,000.degree. C., in an Ar
atmosphere or in vacuum, and then rapidly cooled to give a soft .beta.
single phase having bcc structure. The titanium alloy thus treated is then
subjected to cold working at a percent working .eta. of not less than 20%
to form races. The titanium alloy is then formed into a race. Referring to
the method for forming race, the titanium alloy is subjected to near net
shaping (semi-finished shaping) to minimize the number of steps required
for grinding. Accordingly, the titanium alloy is preferably subjected to
cold working by cold rolling forging. The titanium alloy thus cold-worked
is then subjected to aging at a temperature of from 400.degree. C. to
550.degree. C. for 5 to 10 hours. In this manner, a race material having
.alpha. phase deposited uniformly and finely in .beta. crystalline grains
can be produced. The race material thus obtained can be then subjected to
a predetermined finishing such as grinding to finally obtain a race made
of .beta. type titanium alloy.
As mentioned above, a .beta. type titanium alloy exhibits an excellent
cold-workability. Thus, the kind of .beta. type titanium alloy to be used
in the present invention is not specifically limited. However, even an
alloy belonging to .beta. type titanium alloy is liable to instabilization
of residual .beta. phase depending on its alloy composition. If subjected
to cold working, such a .beta. type titanium alloy can form a work-induced
martensite. However, the foregoing work-induced martensite can crack if
the percent cold working .eta. is great. Accordingly, among .beta. type
titanium alloys, a .beta. type titanium alloy which hardly forms such a
work-induced martensite is preferably used. In particular, a Ti-Mo-based
.beta. type titanium alloy such as Ti-15Mo-5Zr and Ti-15Mo-5Zr-3Al is
preferably used for positions requiring corrosion resistance.
THIRD EMBODIMENT
In the rolling bearing according to the third embodiment of the present
invention, at least one of the inner race and the outer race is formed by
a .beta. type titanium alloy, the percent cold working is predetermined to
a range of from 5 to 20%, and the cold working is followed by shot
peening.
In the third embodiment of the present invention, as shown in FIG. 2, a
titanium alloy is subjected to solution treatment, and then rapidly cooled
in the same manner as in the second embodiment of the present invention.
The titanium alloy thus treated is then subjected to cold working such as
cold rolling forging. The titanium alloy is then subjected to shot
peening. The titanium alloy is then finally subjected to aging to produce
a rolling bearing having a surface hardness Hv of not less than 600.
The reason why a .beta. type titanium alloy which has been subjected to
shot peening has a hardened surface layer will be described hereinafter.
In other words, the shot peening of .beta. single phase texture obtained by
rapidly cooling the solution-treated titanium alloy causes the surface
layer to undergo plastic deformation that causes the introduction of a
large amount of dislocation. When the titanium alloy thus treated is then
aged, hard .alpha. phase is deposited in the plastically-deformed surface
layer with a high density dislocation as nucleating site. Thus, the shot
peening causes the surface layer to have more nucleating sites at which
a-phase is deposited than the core which undergoes not plastic
deformation. As a result, a phase is finely and uniformly deposited in the
surface layer similarly to cold working, drastically hardening the surface
layer alone.
However, as mentioned above, if a .beta. type titanium alloy material which
has been subjected to solution treatment is then directly subjected to
shot peening, the work strain thus provided and its depth are limited,
limiting the rise in the surface hardness.
Thus, in the third embodiment of the present invention, a titanium alloy is
subjected to cold working at a percent working of from 5 to 20% before
shot peening to obtain a rolling bearing having a good toughness as well
as a surface hardness Hv of not less than 600.
The reason why the percent cold working is predetermined to a range of from
5 to 20% will be described hereinafter.
In other words, if a titanium alloy is subjected to cold working, there is
a fear that the metallic texture is hardened to the core to thereby impair
its toughness. Therefore, in order to obtain a good toughness, it is
preferred that a titanium alloy be not subjected to cold working or be
subjected to cold working at a low percent working. However, if the
percent cold working falls below 5%, a titanium alloy exhibits a surface
hardness Hv as small as not more than 600 even when subjected to shot
peening and thus cannot provide a surface hardness required for rolling
bearing. On the contrary, if the percent cold working exceeds 20%, a
titanium alloy exhibits a remarkably reduced toughness. Accordingly, in
the present embodiment, the percent cold working is predetermined to a
range of from 5 to 20%.
If a titanium alloy is subjected to cold working at a percent working of
from 5 to 20%, followed by shot peening, it is provided with a work strain
in the surface layer as much as obtained when it is subjected to cold
working at a high percent working. When the titanium alloy is then
subjected to aging, its core undergoes aged hardening to an extent such
that the toughness thereof is not impaired, and the micro-deposition of
hard .alpha. phase in the surface layer proceeds to cause hardening.
Thus, in accordance with the third embodiment of the present invention, a
rolling bearing suitable for use in working atmospheres requiring
toughness can be obtained.
FIG. 3 is a chart illustrating a modification of the third embodiment of
the present invention. In this modification, a titanium alloy which has
been subjected to aging is again subjected to shot peening.
Shot peening originally exerts an effect of applying residual compression
stress to the surface layer to enhance its fatigue strength.
Shot peening after cold working can enhance the surface hardness of a
titanium alloy. However, since work strain which has been given by shot
peening can be released during a prolonged heating and storage at the
aging step, the residual compression stress is reduced after the
termination of aging, possibly making it impossible to enhance the fatigue
strength of the titanium alloy.
Thus, in this modification, a titanium alloy which has been subjected to
aging is again subjected to shot peening as shown in FIG. 3 so that the
surface layer thereof is provided with a high residual compression stress
to enhance the fatigue strength thereof.
In other words, if a .beta. type titanium alloy is used as a race material,
even when residual .beta. phase is subjected to plastic deformation, the
residual .beta. phase which has been aged has .beta. phase-stabilizing
elements in a high concentration to show a high degree of stabilization of
.beta. phase. Thus, unlike steel material such as stainless steel, the
.beta. type titanium alloy does not undergo work-induced martensite
transformation. However, since the residual .beta. phase exhibits a very
great plastic transformability, it can have a large amount of work strain
accumulated therein as compared with steel materials when subjected to
shot peening. As a result, the .beta. type titanium alloy can be provided
with a high residual compression stress, making it possible to enhance the
fatigue strength thereof.
In accordance with this modification, a rolling bearing suitable for use in
working atmospheres particularly requiring excellent fatigue life and
fatigue strength can be obtained.
FOURTH EMBODIMENT
In the rolling bearing according to the fourth embodiment of the present
invention, at least one of the inner race and the outer race is formed by
a .beta. type titanium alloy, the percent cold working is predetermined to
not less than 20%, and the content of residual .beta. phase in the .beta.
type titanium alloy is predetermined to a range of from 30 to 80 vol %.
When the rolling bearing operates with a lubricant contaminated by foreign
matters, impressions are formed by the foreign matters on the surface
layer of the race, possibly reducing the bearing life. Thus, when a steel
material such as stainless steel is used, the following countermeasure is
taken. As previously mentioned, the edge of the impressions are allowed to
undergo plastic deformation when they repeatedly come in contact with the
rolling elements which pass thereby during the period between the
formation of the impressions and the generation of cracks in the edge of
the impressions so that the concentration of stress on the edge of the
impressions is relaxed, making it possible to prolong the life of bearing
when the lubricant is contaminated by foreign matters.
In other words, residual austenite contained in steel materials is a soft
texture liable to plastic deformation. When a high stress is concentrated
on the edge of impressions formed by foreign matters which have entered in
the lubricant on the surface layer of a race made of steel material, the
edge of the impressions can easily undergo plastic deformation as well as
stress-induced transformation so that it is transformed to a hard
martensite texture. As a result, the edge of the impressions shows a
hardness rise. When the drop of concentration of stress and the hardness
rise are balanced, the edge of the impressions no longer undergoes plastic
deformation. To be short, when a race made of a steel material operates
with a lubricant contaminated by foreign matters, the residual austenite
texture exerts an effect of enhancing fatigue strength due to stress
relaxation and martensite transformation to improve the bearing life.
In the case of .beta. type titanium alloy, residual .beta. phase exerts the
same effect as exerted by residual austenite in steel materials. In other
words, a .beta. type titanium alloy is subjected to solution treatment at
a .beta. phase temperature of not lower than .beta. transition, and then
rapidly cooled to give a residual .beta. single phase which normally stays
soft. Subsequently, the titanium alloy is subjected to aging to cause hard
.alpha. phase to be uniformly and finely deposited in the surface layer,
thereby forming an (.alpha.+.beta.) texture and enhancing the surface
hardness.
In other words, the .beta. type titanium alloy forms a two-phase texture
having a hard .alpha. phase deposited in a soft .beta. phase texture.
Thus, when the rolling bearing operates with a lubricant contaminated by
foreign matters, the edge of impressions formed on the soft residual
.beta. phase is allowed to undergo plastic deformation when it repeatedly
comes in contact with the rolling elements passing thereby during the
period between the formation of the impressions and the generation of
cracks in the edge of the impressions, making it possible to relax the
concentration of stress on the edge of the impressions.
Further, unlike steel materials, the .beta. type titanium alloy forms an
(.alpha.+.beta.) two-phase texture when subjected to aging. Thus, .beta.
stabilizing elements are concentrated in .beta. phase to raise the
stability of .beta. phase, preventing martensite transformation during
working and hence causing no enhancement of the hardness of the periphery
of the impressions.
In other words, since the residual .beta. phase in the .beta. type titanium
alloy exhibits an extremely high transformability, it can repeatedly form
impressions therein. As a result, the impressions can easily undergo
plastic deformation to relax stress concentration thereon even when they
come in contact with the rolling elements passing thereby. Further, the
.beta. type titanium alloy exhibits a smaller work-hardening index n (see
the equation (2) in the second embodiment) than steel material. By making
the best use of the characteristics, the .beta. type titanium alloy
undergoes no extreme hardening even when repeatedly subjected to plastic
deformation that causes the introduction of a large amount of strain and
thus is little liable to cracking, making it possible to improve the life
of the bearing which operates with a lubricant contaminated by foreign
matters.
In the fourth embodiment of the present invention, too, if a titanium alloy
is merely subjected to solution treatment and aging, it cannot be provided
with a surface hardness Hv required for bearing. Thus, the titanium alloy
which has been subjected to solution treatment followed by rapid cooling
needs to be subjected to cold working similarly to the second and third
embodiments.
The residual .beta. phase, percent cold working .eta., and aging
temperature will be described hereinafter.
(1) Residual .alpha. Phase
As mentioned above, the presence of residual .beta. phase is effective for
the prevention of reduction of the life of bearing even when the lubricant
is contaminated by foreign matters. If the content of residual .beta.
phase falls below 30 vol %, the proportion of residual .beta. phase in the
bearing material is too small to provide a stably prolonged bearing life
when the lubricant is contaminated by foreign matters. On the contrary,
since the residual .beta. phase is soft, if the content of residual .beta.
phase is too great, the resulting rolling bearing exhibits an insufficient
hardness and thus cannot operate over a desired life. To be short, if the
content of residual .beta. phase exceeds 80 vol %, the amount of .alpha.
phase deposited in the surface layer of the .beta. type titanium alloy is
too small to provide a sufficient surface hardness at the initial stage of
aging. As a result, even after aging, a desired surface hardness cannot be
obtained, making it impossible to provide a desired bearing life.
Accordingly, the volumetric proportion of residual .beta. phase needs to
be from 30 to 80 vol %.
The volumetric proportion of residual .beta. phase can be obtained by
removing the surface layer of an alloy material by a depth of about 50
.mu.m by means of chemical polishing (e.g., with an aqueous solution of
hydrofluoric acid and hydrogen peroxide), and then quantitatively
analyzing the surface exposed by means of x-ray diffraction.
(2) Percent Cold Working .eta.
As mentioned in the second embodiment of the present invention, if a
titanium alloy which has been subjected to solution treatment is subjected
to cold working such as cold rolling forging, it can exhibit an enhanced
surface hardness HRC or strength when subjected to aging. In other words,
cold working causes dislocation to be uniformly introduced into the
crystalline grains. Thus, .alpha. phase is uniformly and finely deposited
in .beta. crystalline grains with the dislocation as a nucleus production
ground, making it possible to enhance surface hardness HRC and strength.
Thus, it is normally necessary that the percent cold working .eta. be not
less than 20%, preferably not less than 30%, similarly to the second
embodiment of the present invention.
If an emphasis is placed on toughness, the percent cold working .eta. is
preferably predetermined to a range of from 5 to 20% on condition that the
cold working is followed by shot peening as mentioned in the third
embodiment.
(3) Aging Temperature
A titanium alloy which has been subjected to cold working needs to be
subjected to aging for hardening. If the aging temperature falls below
400.degree. C, .omega. phase is preferentially deposited. This .omega.
phase remarkably hardens the surface layer but exerts an embrittling
effect. Thus, the deposition of this .omega. phase needs to be avoided as
much as possible. On the contrary, if the aging temperature exceeds
550.degree. C., hard .alpha. phase can be deposited in the surface layer
in a short period of time. However, grain boundary reaction type
deposition becomes dominant, causing .alpha. phase to be preferentially
deposited in layer at the residual .beta. phase grain boundary. As a
result, coarse acicular .alpha. phase is deposited in .beta. grains,
constituting a hindrance to surface hardening. In order to enhance surface
hardness, it is preferred that the aging temperature be lowered. However,
the aging time is prolonged. Accordingly, the aging temperature is
preferably predetermined to a range of from 450.degree. C. to 500.degree.
C.
The present invention will be further described in the following examples,
but the present invention should not be construed as being limited
thereto.
FIRST GROUP OF EXAMPLES
The inventors prepared disc-shaped specimens as races made of various
titanium alloys and various steel materials.
Table 1 shows the name of the material of various specimens, the surface
hardening method, the solution treatment conditions (or hardening
conditions), and the aging conditions (or tempering conditions).
TABLE 1
Solution Aging
Treatment Conditions
Surface Conditions (or
Race Name Hardening (or Hardening Tempering
No. of Material Method Conditions) Conditions)
A Ti-6Al-4V 850.degree. C./10 hr. 950.degree. C. water
540.degree. C./
gaseous cooling 4 hr.
nitriding
B Ti-6Al-2Sn- " 910.degree. C. oil 590.degree. C./
4Zr-6Mo cooling 4 hr.
C Ti-15Mo-5Zr " 730.degree. C. water 500.degree. C./
cooling 16.7 hr.
D Ti-15Mo-5Zr- " 735.degree. C. water 450.degree. C./
3Al cooling 16.7 hr.
E Ti-15V-3Cr- " 800.degree. C. water 450.degree. C./
3Sn-3Al cooling 6 hr.
F Ti-10V-2Fe- " 760.degree. C. water 400.degree. C./
3Al cooling 8 hr.
G Ti-0.3Mo- " 700.degree. C. --
0.8Ni annealing
H Ti-5Ta " 700.degree. C. --
annealing
I Pure titanium " 700.degree. C. --
(JIS3) annealing
J SUS630H Immersion 1,050.degree. C. oil 500.degree. C./
hardening cooling 1 hr.
K SUS440C " 1,050.degree. C. oil 180.degree. C./
cooling 2 hr.
L SCR420 930.degree. C./4 hr. 850.degree. C. oil 180.degree.
C./
Carburizing cooling 2 hr.
M SUJ2 Immersion 850.degree. C. oil 180.degree. C./
hardening cooling 2 hr.
The races A and B were made of (.alpha.+.beta.) type titanium alloys, the
races C to F were made of .beta. type titanium alloys, the races G and H
were made of .alpha. type titanium alloys, the race I was made of pure
titanium (JIS3), and the races J to M were made of predetermined steel
materials.
The races A to H, which had been made of titanium alloys, and the race I,
which had been made of pure titanium, were subjected to gaseous nitriding
at a temperature of 850.degree. C. as surface treatment, and then cooled
with nitrogen. The races A to F were subjected to solution treatment at a
temperature of from 730 to 950.degree. C. while being subjected to water
cooling or oil cooling, and then subjected to aging at a temperature of
from 450 to 590.degree. C. for 4 to 10 hours to undergo hardening. On the
other hand, the races G to I were subjected to gaseous nitriding, and then
subjected to annealing at a temperature of 700.degree. C.
The races J, K and M were subjected to immersion hardening at a temperature
of from 850 to 1,050.degree. C., and then subjected to tempering at a
temperature of from 180 to 500.degree. C. for 1 to 2 hours.
The race L was subjected to carburizing at a temperature of 930.degree. C.
for 4 hours, subjected to hardening at a temperature of 850.degree. C.,
and then subjected to tempering at a temperature of 180.degree. C. for 2
hours.
Table 2 shows the surface hardness HRC of the races, the results of salt
spray corrosion test on these races, and the results of submerged life
test on rolling bearings having rolling elements made of Si.sub.3 N.sub.4.
TABLE 2
Material Results of Sub-
of Surface Salt Spray merged
Rolling Hardness Corrosion Life L.sub.10 (.times.
Example No. Race No. Elements (HRC) Test 10.sub.6 Cycle)
Example 1 A Si.sub.3 N.sub.4 58.1 Good 25.3
Example 2 B " 58.3 Good 29.4
Example 3 C " 60.2 Good 33.4
Example 4 D " 60.0 Good 31.5
Example 5 E " 59.8 Good 28.3
Example 6 F " 58.1 Good 24.8
Comparative G " -46.2 Good 3.8
Example 101
Comparative H " 46.5 Good 2.8
Example 102
Comparative I " 38.7 Good 3.6
Example 103
Comparative J Si.sub.3 N.sub.4 43.0 Fair 2.9
Example 104
Comparative K " 59.7 Poor 2.5
Example 105
Comparative L " 62.1 Poor 1.4
Example 106
Comparative M " 62.0 Poor 1.3
Example 107
The .beta. type titanium alloy used in Comparative Examples 101 and 102 and
pure titanium used in Comparative Example 103 don't undergo hardening when
subjected to heat treatment. Thus, all these races exhibit a surface
hardness as low as not more than 47, making it impossible to provide a
surface hardness sufficient for bearing.
On the contrary, the (.alpha.+.beta.) type titanium alloy used in Examples
1 and 2 and the .beta. type titanium alloy used in Examples 3 to 6 exhibit
a surface hardness HRC of not less than 57 when subjected to heat
treatment, making it possible to provide a surface hardness sufficient for
bearing. Thus, a race which exhibits an excellent seizing resistance and
thus is not liable to adhesive abrasion can be obtained.
For the salt spray corrosion test, a 5% aqueous solution of NaCl was used.
The 5% aqueous solution of NaCl was sprayed onto the various races A to M
at a temperature of 35.degree. C. for 150 hours. After spraying, corrosion
products were removed from these races A to M. The change in the weight of
these races A to M was then determined. From these measurements, the
corrosion rate per year was calculated, and the saline resistance was then
evaluated. Referring to criterion for evaluation, when the corrosion rate
is not more than 0.13 mm/year, the corrosion resistance is rated as
"good". When the corrosion rate is from 0.13 to 1.3 mm/year, the corrosion
resistance is rated as "fair (slightly poor)". When the corrosion rate is
not less than 1.3 mm/year, the corrosion resistance is rated as "poor
(unacceptable)".
Table 2 shows that all Comparative Examples 104 to 107, which comprise
races made of steel material, corrode remarkably with rust and thus
exhibit an insufficient corrosion resistance while Examples 1 to 6 and
Comparative Examples 101 to 103, which comprise races made of titanium
alloy, give good test results. In other words, concerning the saline
resistance, the races made of steel material didn't give satisfactory
results while the races made of titanium alloy, that is, not only .beta.
type titanium alloy or (.alpha.+.beta.) type titanium alloy but also
.alpha. type titanium alloy or pure titanium, gave satisfactory results.
The submerged life test will be described hereinafter.
FIG. 4 is a schematic diagram illustrating the structure of a submerged
thrust bearing life testing machine for use in the submerged life test.
The various races (A to M) and the rolling elements made of Si.sub.3
N.sub.4 were assembled into a thrust ball bearing 1. For the submerged
life test, the thrust ball bearing 1 was immersed in the water in a
testing tank 2. A rotary axis 7 was then allowed to rotate while the
bearing was under a predetermined test load applied from the lower side.
In FIG. 4, the reference numeral 3 indicates an inner race, the reference
numeral 4 indicates an outer race, the reference numeral 5 indicates a
ball, and the reference numeral 6 indicates a cage. As the water which
fills the testing tank 2 there was used tap water. The tap water was
supplied from the lower side of the testing tank 2, and then overflown
from the upper side of the testing tank 2.
The submerged life test conditions will be described hereinafter.
Test Conditions
Bearing tested: Thrust ball bearing (Designation No. 51305)
Rotary speed of rotary axis: 1,000 rpm
Test load: 150 kgf
Material of rolling elements: Si.sub.3 N.sub.4
Material of cage: Fluororesin
The inner race and outer race in each bearing to be used in the submerged
life test were prepared from the same material, which is indicated in
Table 3.
The submerged life L.sub.10 indicates the time at which 10% of the
specimens show a vibration level of 5 times the initial value as detected
by an acceleration pick up sensor. The submerged life is quantitatively
evaluated by the number of rotations cumulated until this point is
reached.
Table 2 shows that Comparative Examples 101 to 107 exhibit an extremely
short submerged life L.sub.10. This is probably because Comparative
Examples 101 and 102 and Comparative Example 103 use .alpha. type titanium
alloys and pure titanium, respectively, and thus exhibit a reduced
strength and a reduced surface hardness HRC and hence undergo early
flaking due to surface fatigue. In Comparative Examples 104 to 107, the
races were made of alloy steel and thus undergo remarkable corrosion
abrasion and exhibit an extremely short bearing life.
On the contrary, in Examples 1 to 6, the races were made of a .beta. type
titanium alloy or (.alpha.+.beta.) type titanium alloy. Combined with
rolling elements made of Si.sub.3 N.sub.4, these races exhibit a
remarkably prolonged submerged life L.sub.10.
SECOND GROUP OF EXAMPLES
The inventors prepared rolling elements made of SUS440C and SUJ2. Combined
with these rolling elements, the races A to M set forth in Table 1 were
subjected to salt spray corrosion test and submerged life test in the same
manner as mentioned above.
Table 3 shows the combination of races and rolling elements and the results
of the various tests on these combinations.
TABLE 3
Material Results of
of Surface Salt Spray Submerged
Race Rolling Hardness Corrosion Life L.sub.10 (.times.
Example No. No. Elements (HRC) Test 10.sup.6 Cycle)
Example 11 A SUS440C 58.1 Good 12.3
Example 12 B " 58.3 Good 13.0
Example 13 C " 60.2 Good 15.9
Example 14 D " 60.0 Good 16.5
Example 15 E " 59.8 Good 15.6
Example 16 F " 58.1 Good 14.2
Comparative A SUJ2 58.1 Good 1.2
Example 111
Comparative B " 58.3 Good 1.0
Example 112
Comparative C " 60.2 Good 0.9
Example 113
Comparative D " 60.0 Good 1.2
Example 114
Comparative E SUJ2 59.8 Good 1.5
Example 115
Comparative F " 58.1 Good 1.3
Example 116
Comparative G SUS440C 46.2 Good 3.8
Example 117
Comparative H " 46.5 Good 2.8
Example 118
Comparative I " 38.7 Good 3.6
Example 119
Comparative K " 59.7 Poor 2.5
Example 120
As can be seen in Comparative Examples 111 to 116, if SUJ2 (high carbon
chromium bearing steel) is used as rolling element material, even when the
race is made of .beta. type titanium alloy or (.beta. +.beta.) type
titanium alloy, the resulting rolling bearing exhibits a reduced submerged
life L.sub.10. This is because titanium alloy and SUJ2 greatly differ
electronegatively from each other to cause galvanic corrosion that attacks
and drastically wears the rolling elements made of SUJ2, which is
electronegatively greater than titanium alloy.
In Comparative Examples 117 to 119, races made of .alpha. type titanium
alloy or pure titanium and rolling elements made of SUS440C were combined.
However, the .alpha. type titanium alloy or pure titanium used in the
races exhibits a deteriorated strength and surface hardness. The resulting
surface fatigue causes early flaking that reduces the submerged life
L.sub.10. In Comparative Example 120, a race made of SUS440C and rolling
elements made of SUS440C were combined. However, this combination
accelerates the corrosion, deteriorating both the submerged bearing life
and saline resistance.
On the contrary, Examples 11 to 16 concern a combination of race made of
.beta. type titanium alloy or (.alpha.+.beta.) type titanium alloy and
rolling elements made of SUS440C. These combinations exhibit a reduced
submerged bearing life as compared with the case where the race is made of
Si.sub.3 N.sub.4 (see Table 2). However, since there is little difference
in electronegativity between titanium alloy and SUS440C, the progress of
galvanic corrosion is inhibited, making it possible to secure some
submerged bearing life.
As can be seen in the foregoing first and second groups of examples, the
combination of (.alpha.+.beta.) type or .beta. type titanium alloy as race
material and Si.sub.3 N.sub.4 as rolling element material is most suitable
for corrosion resistance. It is also made obvious that even rolling
elements made of SUS440C can provide a sufficient bearing life in water or
sea water.
THIRD GROUP OF EXAMPLES
The inventors prepared combined angular ball bearings from various titanium
alloys and steel materials. The change in the bearing clearance and the
expansion of the inner race during high speed rotation were then
calculated. The rise in the temperature of the outer race was measured.
Table 4 shows various bearing materials used in Examples 21 and 22 and
Comparative Examples 131 to 136, the solution treatment conditions
(hardening conditions) and the aging conditions (or tempering conditions).
TABLE 4
Inner
Race
Inner Race Solution Aging
Material Treatment Conditions
Conditions
Outer Inner Rolling (or Hardening (or
Tempering
Race Race Elements Conditions)
Conditions)
Example 21 SUJ2 Ti-6Al-4V Si.sub.3 N.sub.4 900-950.degree. C.
water cooling 500-540.degree. C./4 hr.
Example 22 SUJ2 Ti-22V-4Al Si.sub.3 N.sub.4 750-800.degree. C.
water cooling 450-500.degree. C./4 hr.
Comparative SUJ2 SUS440C Si.sub.3 N.sub.4 1050.degree. C. oil
cooling 180.degree. C./2 hr.
Example 131
Comparative SUJ2 SUJ2 Si.sub.3 N.sub.4 840.degree. C. oil
cooling 180.degree. C./2 hr.
Example 132
Comparative SUJ2 Ti-6Al-4V SUJ2 900-950.degree. C. water
cooling 500-540.degree. C./4 hr.
Example 133
Comparative SUJ2 Ti-22V-4Al SUJ2 750-800.degree. C. water
cooling 450-500.degree. C./4 hr.
Example 134
Comparative Ti-6Al-4V Ti-6Al-4V Si.sub.3 N.sub.4 900-950.degree. C.
water cooling 500-540.degree. C./4 hr.
Example 135
Comparative Ti-22V-4Al Ti-22V-4Al Si.sub.3 N.sub.4 750-800.degree. C.
water cooling 450-500.degree. C./4 hr.
Example 136
The inner races of Example 22 and Comparative Examples 134 and 136 were
made of .beta. type titanium alloy, and the inner races of Example 21 and
Comparative Examples 133 and 135 were made of (.alpha.+.beta.) type
titanium alloy. These materials were each subjected to solution treatment
and aging under conditions set forth in Table 4.
The inner races of Comparative Examples 131 and 132 were made of alloy
steel. The alloy steel was subjected to hardening at a predetermined
temperature, and then subjected to tempering at a predetermined
temperature.
The inner races made of titanium alloy were coated with TiN on the raceway
track to secure sufficient abrasion resistance and seizing resistance.
The rolling bearings of Examples 21 and 22 and Comparative Examples 131 to
136 were measured for change in the bearing clearance and expansion of the
inner race during high speed rotation using a high speed rotary testing
machine shown in FIG. 5. The rise in the temperature of the outer race was
then determined. In FIG. 5, the reference numeral 12 indicates an outer
race, the reference numeral 13 indicates an inner race, and the reference
numeral 14 indicates rolling elements.
In other words, the outer race 12 was incorporated in a housing 15, and the
inner race 13 was put on a rotary axis 16 so that a back-to-back type
combined angular ball bearing 11 was mounted in the high speed rotary
testing machine. The rotary axis 16 was then rotated. The temperature of
the outer race 12 was then measured by means of a thermocouple 17 inserted
in the housing 15.
The test conditions will be described hereinafter.
High Speed Test
Bearing tested: Back-to-back type angular ball bearing (Designation No.
7013C)
Preload during mounting: 10 kgf
Lubrication: Grease
Grease used: Isoflex NBU15 (produced by NOK Kluber Co., Ltd.)
Rotary speed of rotary axis: 12,000 rpm
Table 5 shows the results of high speed rotary test.
TABLE 5
Linear Density Tempera-
Expansion of ture
Coefficient Inner Density of Difference
Rise in
of Inner Race Rolling Between Change in
Inner
Race Mate- Element Inner Race Bearing Expansion
Tace
Material rial Material and Outer Clearance of Inner
Tempera-
Example No. (/.degree. C.) (g/cm.sup.3) (g/cm.sup.3) Race(.degree. C.)
(.mu.m) Race (.mu.m) ture (.degree. C.)
Example 21 0.0000088 4.43 3.2 7 2.1 2.9
8.7
Example 22 0.0000085 4.69 3.2 7 2.8 3.0
8.5
Comparative 0.0000101 7.70 3.2 7 -0.7 4.5
10.8
Example 131
Comparative 0.0000125 7.83 3.2 7 -8.1 5.1
12.4
Example 132
Comparative 0.0000088 4.43 7.83 7 2.1 2.9
11.4
Example 133
Comparative 0.0000085 4.69 7.83 7 2.8 3.0
11.2
Example 134
Comparative 0.0000088 4.43 3.2 7 -5.7 4.9
11.6
Example 135
Comparative 0.0000085 4.69 3.2 7 -5.5 5.1
11.5
Example 136
For the evaluation of the bearing clearance, the change developed when the
temperature difference between the inner race and the outer race reaches
7.degree. C. was determined.
In Comparative Example 131, the outer race was made of SUJ2, the inner race
was made of SUS440C, and the rolling elements were made of Si.sub.3
N.sub.4. Since SUS440C exhibits a greater linear expansion coefficient
than titanium alloy, the bearing clearance is reduced with the temperature
difference between the inner race and the outer race being 7.degree. C.
Further, since SUS440C has a great density, it exhibits a great expansion
due to centrifugal force, causing a great rise in the temperature of the
outer race. In Comparative Example 132, both the inner race and the outer
race were made of SUJ2, and the rolling elements were made of Si.sub.3
N.sub.4. Since both the inner race and the outer race were made of SUJ2,
the bearing clearance showed a remarkable drop, and the expansion of the
inner race and the rise in the temperature of the outer race were raised.
In Comparative Examples 133 and 134, the outer race was made of SUJ2, the
inner race was made of titanium alloy, and the rolling elements were made
of Si.sub.3 N.sub.4. Since the inner race was made of titanium alloy, the
bearing clearance showed a rise rather than drop. The expansion of the
inner race was small. However, the outer race showed a great temperature
rise. This is probably because the rolling elements are made of SUJ2,
which has a greater density than ceramics, and thus is given a great
centrifugal force, resulting in the rise in the friction between the track
on the race and the rolling surface of the rolling elements.
In Comparative Examples 135 and 136, both the inner race and the outer race
were made of titanium alloy, and the rolling elements were made of
Si.sub.3 N.sub.4. Since the inner race and the outer race was made of the
same material, the bearing clearance shows a drop and the expansion of the
inner race is raised if evaluated with the temperature difference between
the inner race and the outer race being 7.degree. C. As a result, the rise
in the temperature of the outer race is raised. Accordingly, taking into
account the high speed rotary operation, the inner race and the outer race
should not be made of the same material. However, since titanium alloy
exhibits a smaller linear expansion coefficient than SUJ2, the reduction
of the bearing clearance can be less than Comparative Example 132 in which
both the inner race and the outer race are made of SUJ2. Accordingly, the
rise in the temperature of the outer race can be inhibited more than in
Comparative Example 132.
On the contrary, in Examples 21 and 22, the inner race was made of titanium
alloy, and the rolling elements were made of Si.sub.3 N.sub.4. Even if
there occurs a temperature difference of 7.degree. C. between the inner
race and the outer race, the bearing clearance does not show a drop but
increases. The expansion of the inner race due to centrifugal force is far
less than in Comparative Examples 131 to 136. Thus, the rise in the
temperature of the outer race during high speed rotation can be reduced to
not more than 10.degree. C. Accordingly, the rolling bearings according to
these examples are suitable for high speed rotation.
As can be seen in the present group of examples, a combination of an inner
race made of titanium alloy, an outer race made of steel material such as
SUJ2 and rolling elements made of Si.sub.3 N.sub.4 is optimum for bearing
for use in machines which operate at a high rotary speed such as machine
tool.
FOURTH GROUP OF EXAMPLES
The inventors prepared a disc-shaped specimen from Ti-15V-3Cr-3Sn-3Al as
.beta. type titanium alloy. The specimen was subjected to solution
treatment at a temperature of 850.degree. C. in an Ar atmosphere,
water-cooled, and then subjected to cold rolling (cold working) at various
percent cold working .eta.. The specimen was subjected to aging at a
temperature of 450.degree. C. for 5 to 8 hours, and then measured for
surface hardness Hv by means of a Vickers hardness testing machine.
FIG. 6 is a characteristic curve illustrating the relationship between
percent cold working .eta. and Vickers hardness Hv after aging.
There is a relationship represented by the following equation (3) between
surface hardness Hv (Vickers hardness) and surface hardness HRC (Rockwell
C hardness).
Hv=10HRC+30 (3)
Accordingly, in order to obtain a surface hardness of not less than 57 as
calculated in terms of HRC, it is necessary that the surface hardness Hv
be not less than 600 according to the equation (3).
However, as evident from FIG. 6, if the percent cold working .eta. is less
than 20%, the surface hardness Hv after aging is not more than 600, making
it impossible to obtain a sufficient hardness. On the contrary, if the
percent cold working .eta. is not less than 20%, the surface hardness Hv
after aging is not less than 600, making it possible to obtain a bearing
material having a sufficient hardness. Further, if the percent cold
working .eta. is not less than 30%, a bearing material having a stabilized
hardness of not less than 600 can be obtained.
The same disc-shaped specimen as used above (.beta.-titanium alloy,
Ti-15V-3Cr-3Sn-3Al) was subjected to solution treatment, water cooling,
cold working, etc. in the same manner as mentioned above. The specimen was
then subjected to aging under isothermal conditions (450.degree. C.) for 5
to 50 hours. The specimen was then measured for surface hardness Hv. The
specimen was also subjected to submerged life test in the same manner as
in the first group of examples.
Table 6 shows the results of measurement of hardness Hv and submerged life
L.sub.10 vs. percent cold working .eta..
TABLE 6
Sub-
Percent merged
Cold Aging Life
Working Time Hardness L.sub.10 (.times.
Example No. .eta. (%) (hr) (Hv) 10.sup.6 cycle)
Example 41 25 5 618 16.8
Example 42 30 5 623 17.1
Example 43 50 5 629 17.1
Example 44 80 5 631 18.9
Example 45 25 7 620 17.3
Example 46 30 7 622 18.5
Example 47 50 7 631 19.7
Example 48 80 7 638 20.1
Comparative Example 141 25 50 583 4.2
Comparative Example 142 30 50 585 4.4
Comparative Example 143 50 50 590 4.5
Comparative Example 144 80 50 597 4.8
Comparative Example 145 0 5 424 1.2
Comparative Example 146 0 7 455 1.6
Comparative Example 147 0 10 451 1.4
Comparative Example 148 0 50 448 1.4
Comparative Example 149 15 5 568 4.3
Comparative Example 150 15 7 572 5.0
Comparative Example 151 15 10 572 5.1
As can be seen in Table 6, in Comparative Examples 141 to 144, cold working
was effected at a percent working .eta. of from 25 to 80%. In other words,
cold working was effected at a percent working .eta. of not less than 20%.
However, since aging was effected for period of time as long as 50 hours,
the bearing materials were softened and thus exhibited a reduced surface
hardness Hv and submerged life L.sub.10. This is probably because the
aging time T is too long, giving overaging that causes hard .alpha. phase
to grow coarsely or .alpha. phase to be deposited at grain boundary and
hence causing a hardness drop. In Comparative Examples 145 to 151, cold
working was effected at a percent working .eta. of not more than 20%,
making it impossible to obtain satisfactory results for use in special
corrosive atmospheres concerning hardness Hv and submerged life L.sub.10.
This is probably because if the percent cold working .eta. is low,
dislocation is nonuniformly introduced, making it difficult for .alpha.
phase to be uniformly and finely deposited in .beta. crystalline grins.
Thus, the resulting degree of reinforcement is small. Further, .alpha.
phase is preferentially deposited at grain boundary to reduce the grain
boundary strength, causing early flaking. On the contrary, in Examples 41
to 48, the percent cold working .eta. is not less than 20%, and the aging
time T is as short as 5 to 7 hours, making it possible to obtain a
hardness Hv of not less than 600 and hence a sufficient submerged life
L.sub.10. The inventors measured the relationship between percent cold
working .eta. and aging time T (hr) required until the highest hardness is
reached. Table 7 shows the measurements.
TABLE 7
Aging Time (hr)
Percent Cold Required Until
working .eta. Highest Hardness
Example No. (%) is Reached
Example 51 25 5
Example 52 30 5
Example 53 50 4
Example 54 70 4
Comparative Example 161 0 7
Comparative Example 162 15 6
As can be seen in Table 7, in Comparative Example 161, no cold working is
effected, requiring 7 hours of aging time T until the highest hardness is
reached. In Comparative Example 162, the percent cold working .eta. is as
low as 15%, requiring 6 hours of aging time T. On the contrary, in
Examples 51 to 54, the percent cold working .eta. is not less than 20%,
requiring aging time T as short as 4 to 5 hours. Thus, a great effect of
accelerating the deposition of .alpha. phase in .beta. crystalline grains
can be exerted.
FIFTH GROUP OF EXAMPLES
The inventors examined a bearing material which had been subjected to shot
peening after cold working and a bearing material which had not been
subjected to shot peening after cold working for the relationship between
percent cold working .eta. and surface hardness after aging.
In some detail, Ti-15Mo-5Zr as .beta. type titanium alloy was subjected to
solution treatment at a temperature of 750.degree. C. in an Ar atmosphere,
water-cooled to form a residual .beta. single phase texture, subjected to
cold rolling (cold working) at a predetermined percent working .eta.,
subjected to shot peening using a straight-hydraulic air blast machine,
and then subjected to aging at a temperature of 475.degree. C. for 5 hours
to prepare a specimen. Separately, a specimen was prepared in the same
manner as mentioned above except that the titanium alloy was not subjected
to shot peening after cold working. The cold working was effected at a
percent working .eta. of 0%, 5%, 10%, 15%, 20%, 30%, and 50%,
respectively.
The shot peening conditions will be described below.
Shot Peening Conditions
Shot: Shot intensity 6A
Shooting material: Cast steel
Grain diameter: 400 .mu.m
Surface hardness Hv: 420
These specimens were each measured for surface hardness Hv by means of a
Vickers hardness tester.
FIG. 7 shows a characteristic curve illustrating the relationship between
percent cold working .eta. and surface hardness Hv after aging in the
present examples, wherein .oval-solid. indicates the case where shot
peening is effected after cold working, and .oval-hollow. indicates the
case where only cold working is effected.
As can be seen in FIG. 7, cold working, if not followed by shot peening,
must be effected at a percent working .eta. of not less than 20% to obtain
a bearing material having a surface hardness Hv of not less than 600. On
the contrary, cold working, if followed by shot peening, may be effected
even at a percent working .eta. as low as 5% to obtain a bearing material
having a surface hardness Hv of not less than 600. Further, if the percent
cold working .eta. is low, the bearing material can be prevented from
hardening to the core, making it possible to obtain a good toughness.
The inventors prepared specimens from Ti-15Mo-5Zr as .beta. type titanium
alloy. These specimens were subjected to solution treatment, water
cooling, cold rolling, shot peening and aging. These specimens were then
measured for surface hardness Hv and residual compression stress. These
specimens were also subjected to submerged life test. For comparison,
specimens which had not been subjected to shot peening or cold working
were prepared and subjected to the same tests as mentioned above.
For the measurement of residual compression stress, an X-ray residual
stress meter was used. The measurement conditions will be described
hereinafter.
Conditions for the Measurement of Residual Compression Stress
Target: Cu-K.alpha.
Filter: Ni
Tube voltage: 40 kV
Tube current: 300 mA
For the submerged life test, the same testing machine (see FIG. 4) as used
in the first group of examples was used. The test was effected in the same
manner as in the first group of examples. However, when the specimens were
subjected to shot peening, the race showed a raised surface roughness. In
order to eliminate the effect of this surface roughness, the surface of
these specimens was polished before the submerged life test.
Table 8 shows the measurements of various specimens which had been
subjected to cold working at different percent working .eta..
TABLE 8
Production Conditions Residual
Submerged
Aging Surface Compression
Life L.sub.10
% Cold Shot Condi- Shot Hardness Stress
(.times. 10.sup.6
Example No. Working Peening tions Peening (Hv) (kg/cm.sup.2)
cycle)
Example 61 5 Yes 475.degree. C./5 hr No 603 0
17.1
Example 62 10 Yes " No 625 -2
17.3
Example 63 15 Yes " No 632 -1
17.7
Example 64 25 Yes " No 640 0
18.6
Example 65 30 Yes " NO 644 0
19.5
Example 66 5 Yes " Yes 631 -34
19.7
Example 67 10 Yes " Yes 639 -31
20.1
Example 68 15 Yes " Yes 644 -37
20.6
Example 69 25 Yes " Yes 649 -37
21.3
Example 70 30 Yes " Yes 651 -35
21.5
Comparative 0 No " No 458 0
1.6
Example 171
Comparative 0 Yes " No 521 -2
2.2
Example 172
Comparative 0 Yes " Yes 521 -3.0
4.3
Example 173
Comparative 5 No " No 508 0
4.6
Example 174
Comparative 5 NO " Yes 532 -29
4.8
Example 175
As can be seen in Table 8, in Comparative Example 171, the bearing material
is subjected to neither cold working nor shot peening but aging after
solution treatment. Thus, the resulting specimen exhibits a low surface
hardness Hv and a reduced submerged life L.sub.10.
In Comparative Example 172, shot peening is effected, causing .alpha. phase
to be uniformly and finely deposited in the surface layer. Thus, the rise
in the surface hardness Hv can be recognized as compared with Comparative
Example 171. However, since solution treatment is not followed by cold
working but by shot peening, the resulting specimen exhibits a surface
hardness Hv of not more than 600, making it impossible to provide a
surface hardness Hv required for bearing. In Comparative Example 173, shot
peening is effected after aging as well in addition to the conditions used
in Comparative Example 172, providing a residual compression stress.
However, since no cold working is effected as in Comparative Example 172,
a surface hardness Hv required for bearing cannot be obtained.
In Comparative Example 174, cold working is effected at a percent working
.eta. as low as 5%. However, since no shot peening is effected, a surface
hardness Hv required for bearing cannot be obtained. In Comparative
Example 175, shot peening is effected after aging as well, providing a
residual compression stress. However, since a bearing material which has
been subjected to solution treatment followed by cold working at a low
percent working is not subjected to shot peening before aging as in
Comparative Example 174, a surface hardness Hv required for bearing cannot
be obtained.
On the contrary, in Examples 61 to 70, cold working is effected at a
percent working .eta. of from 5 to 30% before shot peening. Thus, the
resulting specimen exhibits a surface hardness Hv of not less than 600 and
shows a drastic enhancement of submerged life L.sub.10 as compared with
Comparative Examples 171 to 175.
In particular, Examples 66 to 70 involve another shot peening after aging.
Thus, the resulting specimens exhibit a further rise in surface hardness
Hv if the percent cold working .eta. remains the same. Further, the
bearing material can be provided with a residual compression stress. As a
result, the submerged life L.sub.10 can be enhanced.
In Examples 64, 65, 69 and 70, the percent cold working .eta. is
predetermined to not less than 20%. It is thus likely that the bearing
material can be hardened to the core to exhibit a reduced toughness.
However, a surface hardness Hv of not less than 600 can be obtained, and
the submerged life L.sub.10 cannot be reduced. Accordingly, if the rolling
bearing is used in positions requiring toughness, it is preferred that a
bearing material which has been subjected to cold working at a percent
working .eta. of from 5 to 20% followed by shot peening be used. If the
rolling bearing is used in positions where emphasis is placed on surface
hardness rather than toughness, it is preferred that the bearing material
be subjected to cold working at a percent working .eta. of not less than
20% and then directly to aging without shot peening as in the fourth group
of examples. If it is desired to enhance fatigue strength in particular,
it is preferred that the bearing material which has been thus aged be
subjected to shot peening to have a residual compression stress applied
thereto.
SIXTH GROUP OF EXAMPLES
The inventors examined the relationship between aging time T and residual
.beta. phase content and surface hardness Hv and the relationship between
residual .beta. phase content and bearing life when the lubricant is
contaminated by foreign matters.
In some detail, Ti-15V-3Cr-3Sn-3Al as .beta. type titanium alloy was
subjected to solution treatment at a temperature of 800.degree. C. in an
Ar atmosphere, water-cooled to form a residual .beta. single phase,
subjected to cold rolling at a percent working .eta. of 50%, and then
subjected to aging at a temperature of 450.degree. C. for various periods
of time to prepare various specimens composed of (.alpha.+.beta.) texture.
These specimens were then determined for residual .beta. phase content and
measured for surface hardness Hv.
Firstly, the specimen was subjected to chemical polishing with an aqueous
solution comprising 60% hydrogen peroxide and 10% hydrofluoric acid so
that .alpha. processed layer formed on the surface thereof was removed to
a depth of about 50 .mu.m. Subsequently, using an X-ray diffractometer,
the volumetric ratio (vol %) of residual .beta. phase was calculated with
Co-K.alpha. line as a target. As the X-ray diffractometer there was used
Type RAD-III X-ray diffractometer Geiger Flex (produced by Rigaku Corp.).
For the measurement of surface hardness Hv, a Vickers hardness testing
machine was used as in the fourth and fifth groups of examples.
FIG. 8 is a characteristic curve illustrating the relationship between
aging time T and residual .beta. phase content and surface hardness Hv.
As can be seen in FIG. 8, concerning the relationship between aging time T
and surface hardness Hv, as the aging time T increases, the deposition of
.alpha. phase proceeds to reduce the residual .beta. phase content. In
particular, when the aging time T exceeds 1 hour, the volumetric ratio of
residual .beta. phase shows a sudden drop.
On the other hand, concerning the relationship between aging time T and
surface hardness Hv, when the aging time T exceeds 1 hour, and the
deposition of .alpha. phase becomes remarkable, the rise in surface
hardness Hv becomes remarkable. However, when the aging time T exceeds 10
hours, the content of .alpha. phase shows a continuous rise, and the
volumetric ratio of residual .beta. phase continues to drop. Thus, the
surface hardness Hv shows a continuous drop. Accordingly, if aging is
effected for 10 hours or longer, overaging occurs.
The inventors conducted submerged life test on rolling bearings containing
residual .beta. phase which had been aged for different periods of time
shown in FIG. 8 using a submerged thrust bearing life testing machine
shown in FIG. 4.
The conditions for submerged life test will be described hereinafter.
Test Conditions
Bearing tested: Thrust ball bearing (Designation No. 51305)
Rotary speed of rotary axis: 1,000 rpm
Test load: 150 kgf
Material of rolling elements: Si.sub.3 N.sub.4
Material of cage: Fluororesin
Foreign matters: Fe.sub.3 C powder (300 ppm in water)
Grain diameter of foreign matters: 74-147 .mu.m
Surface hardness HRC of foreign matters: 52
The inner race and outer race in each bearing to be used in the submerged
life test were prepared from the same material, which is indicated in
Table 9.
The submerged life L.sub.10 indicates the time at which 10% of the
specimens undergo cracking or flaking which can be observed under
microscope or visually. The submerged life is quantitatively evaluated by
the number of rotations cumulated until this point is reached.
FIG. 9 is a characteristic curve illustrating the relationship between the
residual .beta. phase content and the submerged life L.sub.10 of the
specimens which have been aged for different periods of time as shown in
FIG. 8.
As can be seen in FIG. 9, if the volumetric ratio of residual .beta. phase
falls below 30 vol %, the submerged life L.sub.10 is extremely low,
although the content of hard .alpha. phase is greater than that of
residual .beta. phase. This is because the specimen are overaged. Thus,
.alpha. phase grows coarsely or is deposited at .beta. phase grain
boundary to cause rapid softening. Therefore, the resulting bearing
exhibits an insufficient hardness. Further, since there is a small
residual .beta. phase content, the impressions possibly formed by foreign
matters exert a small effect of relaxing stress. On the other hand, when
the bearing material which has been subjected to solution treatment is
rapidly cooled, a residual .beta. single phase is formed. Therefore, if
the volumetric ratio of residual .beta. phase exceeds 80 vol %, this state
corresponds to that obtained at the initial stage of aging. Thus, the
specimen is not sufficiently hardened. Accordingly, a sufficient surface
hardness Hv cannot be obtained. The submerged life L.sub.10 is extremely
reduced.
On the contrary, if the volumetric ratio of residual .beta. phase falls
within the range of from 30 to 80 vol %, the residual .beta. phase relaxes
stress on the impressions formed by foreign matters even when the
lubricant is contaminated by foreign matters. Further, .alpha. phase is
deposited to an ideal extent, making it possible to provide a surface
hardness Hv of not less than 600 and a stabilized prolonged submerged life
L.sub.10.
The inventors prepared various specimens from Ti-15Mo-5Zr as .beta. type
titanium alloy and Ti-6Al-4V as (.alpha.+.beta.) type titanium alloy.
These titanium alloys were subjected to heat treatment (solution treatment
and aging) under different conditions or cold working at different percent
working .eta.. These specimens were measured for volumetric ratio (vol %)
of residual .beta. phase, surface hardness Hv and submerged life L.sub.10
under the same conditions as mentioned above (lubricant contaminated by
foreign matters).
Table 9 shows the production conditions of these .beta. type titanium
alloys and the measurements of the various specimens.
TABLE 9
Resi- Sub-
dual merged
.beta. life
Solution Aging Aging Surface
phase L.sub.10
Example Treatment % Cold Temp. Time Hardness
(vol- (.times.10.sup.6
No. Alloy Temp. (.degree. C.) Working (.degree. C.) (hr)
(Hv) %) cycle)
Example 71 Ti-15 Mo-5 Zr 750.degree. C. 50 475 3 615
75 9.3
(.beta. type water
titanium) cooling
Example 72 Ti-15 Mo-5 Zr 750.degree. C. 50 475 5 625
59 10.1
(.beta. type water
titanium) cooling
Example 73 Ti-15 Mo-5 Zr 750.degree. C. 50 475 7 630
51 10.4
(.beta. type water
titanium) cooling
Example 74 Ti-15 Mo-5 Zr 750.degree. C. 50 475 10 621
45 9.8
(.beta. type water
titanium) cooling
Example 75 Ti-15 Mo-5 Zr 750.degree. C. 30 475 3 608
78 8.9
(.beta. type water
titanium) cooling
Example 76 Ti-15 Mo-5 Zr 750.degree. C. 30 475 5 611
70 9.2
(.beta. type water
titanium) cooling
Example 77 Ti-15 Mo-5 Zr 750.degree. C. 30 475 7 615
64 9.3
(.beta. type water
titanium) cooling
Example 78 Ti-15 Mo-5 Zr 750.degree. C. 30 475 10 609
58 9.5
(.beta. type water
titanium) cooling
Comparative Ti-15 Mo-5 Zr 750.degree. C. 50 400 5 658
-- 1.3
Example 181 (.beta. type water
titanium) cooling
Comparative Ti-15 Mo-5 Zr 750.degree. C. 50 400 7 666
-- 1.4
Example 182 (.beta. type water
titanium) cooling
Comparative Ti-15 Mo-5 Zr 750.degree. C. 50 550 5 573
50 3.5
Example 183 (.beta. type water
titanium) cooling
Comparative Ti-15 Mo-5 Zr 750.degree. C. 50 550 7 561
45 2.8
Example 184 (.beta. type water
titanium) cooling
Comparative Ti-6 Al-4V 950.degree. C. 0 540 4 421
58 0.8
Example 185 ((.alpha. + .beta.) type water
titanium) cooling
Comparative Ti-6 Al-4V 900.degree. C. 0 540 4 423
43 0.9
Example 186 ((.alpha. + .beta.) type water
titanium) cooling
As can be seen in Table 9, Comparative Examples 181 and 182 provide a
surface hardness Hv of not less than 600 but an extremely short submerged
life L.sub.10. In Comparative Examples 181 and 182, the surface hardness
itself is raised. However, since the aging temperature is as low as
400.degree. C., .omega. phase is formed, reducing the plastic
deformability. Thus, the concentration of stress on the edge of
impressions formed by foreign matters is raised, causing early flaking.
In Comparative Examples 181 and 182, the residual .beta. phase content was
not calculated. This is because the deposition of .omega. phase makes it
impossible to accurately determine the residual .beta. phase content.
However, since .omega. phase is extremely brittle, it has an adverse
effect on the texture even if the volumetric ratio of residual .beta.
phase falls within the range of from 30 to 80 vol %. Accordingly, the
condition for aging so that .omega. phase is deposited even in a slight
amount should be avoided.
In Comparative Examples 183 and 184, the aging temperature is predetermined
too high as 550.degree. C. Thus, .alpha. phase which would be deposited in
layer at residual .beta. phase grain boundary or inside .beta. phase grain
boundary grows coarsely, making it impossible to undergo sufficient aged
hardening.
In Comparative Examples 185 and 186, (.alpha.+.beta.) type titanium alloy
is used as titanium alloy. When subjected to solution treatment followed
by rapid cooling, an (.alpha.+.beta.) type titanium alloy forms a
martensite texture of a (.alpha.+.beta.) two-phase texture which cannot be
subjected to cold working. Accordingly, since this titanium alloy cannot
be subjected to cold working, it exhibits a reduced surface hardness Hv
and an extremely reduced submerged life L.sub.10 even when subsequently
aged.
On the contrary, in Examples 71 to 78, the aging temperature is
predetermined to 475.degree. C., which is the optimum aging temperature
for the present alloy, the aging time is predetermined to a range of from
3 to 10 hours, and the residual .beta. phase content is varied. All these
examples exhibit a surface hardness Hv of not less than 600 and provide a
remarkable improvement of submerged life L.sub.10 as compared with the
comparative examples. Thus, these examples can provide rolling bearings
suitable for use in the conditions where corrosion resistance is required
and foreign matters are incorporated.
As mentioned above, the rolling bearing according to the present invention
comprises an outer race and an inner race and rolling elements which are
provided between the outer race and the inner race such that the rolling
elements rotate freely, characterized in that at least the inner race is
made of a titanium alloy and the rolling elements are made of ceramics.
Thus, the rolling bearing according to the present invention exhibits a
drastically improved corrosion resistance as compared with the case where
the race is made of a steel material such as stainless steel and thus is
suitable for use in corrosive atmospheres such as food machine,
semiconductor producing machine and chemical fiber producing machine which
must be resistant to corrosion with sea water or chemical.
Further, the use of a titanium alloy having a small linear expansion
coefficient and a small density as an inner race material makes it
possible to inhibit the rise in the temperature of the outer race during
high speed rotation and hence provides a rolling bearing suitable for use
in machine tools which operate at a high rotary speed.
Moreover, in accordance with the present invention, the rise in the
production cost can be inhibited as compared with the case where the
bearing is totally made of ceramics.
Further, by forming at least one of the inner race and outer race by a
.beta. type titanium alloy and predetermining the percent cold working
.eta. to not less than 20%, .alpha. phase is deposited in .beta.
crystalline grains to enhance hardness and bearing strength, making it
possible to improve the durability of the bearing.
Moreover, by predetermining the percent cold working .eta. of .beta. 3 type
titanium alloy to a range of from 5 to 20% and subjecting the .beta. type
titanium alloy thus cold-worked to shot peening, .alpha. phase is finely
deposited, enabling drastic rise in the hardness of the surface layer
alone without impairing the toughness. Further, by subjecting the .beta.
type titanium alloy to shot peening after aging as well, the .beta. type
titanium alloy can be provided with a residual compression stress, making
it possible to improve the bearing life in a special atmosphere.
Further, by predetermining the volumetric ratio of residual .beta. phase in
the foregoing .beta. type titanium alloy to a range of from 30 to 80%, the
concentration of stress on the edge of impressions formed on the surface
of the race can be relaxed even when the lubricant is contaminated by
foreign matters, making it possible to provide a rolling bearing which
exhibits an excellent corrosion resistance and a prolonged life even when
the lubricant is contaminated by foreign matters.
While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the
art that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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